1. Field of the Invention
The present invention relates to a device for and a method of concentrating chemical substances for fabrication of a semiconductor device and, more particularly, for treating a sample before the analysis of the chemical substances with a common analytical instrument.
2. Discussion of Related Art
With the high integration of semiconductor devices, the devices are becoming more miniaturized. However, the slightest amount of contaminants contained in the layer material formed on a wafer can have an enormous effect on the characteristics of a semiconductor. An excess of such contaminants may result in a failure in the performance of the semiconductor device. Such failures decrease the yield of the semiconductor device fabrication facility. Therefore, it is necessary to prevent as many contaminants as possible from being introduced to the semiconductor device during fabrication.
A wet cleaning process is typically employed to remove contaminants from the wafer. Chemical substances, for example, fluoric acid, nitric acid, sulfuric acid, oxygenated water and the like are used in the process. These chemical substances must be highly pure to prevent the recontamination of the wafer. To ensure purity, it is necessary to carry out a qualitative analysis and a quantitative analysis to detect even extremely small amounts of contaminants in the chemical substances. Such detection of extremely small quantities of contaminants is called microanalysis. The contaminants that need to be microanalyzed include metals such as iron (Fe), aluminum (Al) and copper (Cu), and ions such as sodium ion (Na.sup.+), ammonium ion (NH.sub.4.sup.+), nitride ion (NO.sub.3.sup.-) and chloride ion (Cl.sup.-). The analyses are performed with analytical instruments such as a graphite furnace atomic absorption spectrometer (GFAAS), an induced coupling plasma mass spectrometer (ICP-MS) and an ion chromatography (IC) analyzer.
To obtain detectable levels of these contaminants for quantitative analysis various distillation methods are employed. Distillation that vaporizes the matrix (i.e., the acids or oxygenated water listed above) of the chemical substances reduces the amount of matrix and thus increase the relative concentration of the impurities contained in the original chemical substances. Distillation processes that concentrate the impurities in a sample of chemical substances as a pretreatment for analysis also typically recover the vaporized matrix, such as the acid, in highly purified form.
Acid of high purity may be obtained through isopiestic distillation systems, vacuum distillation systems, and sub-boiling systems.
The isopiestic distillation, an acid distillation method based on the difference of vapor pressures, is useful for the acids of high vapor pressure such as hydrochloric acid, but not for those of low vapor pressure such as nitric acid and sulfuric acid. It is therefore not suitable for the pretreatment of most analytical samples of interest in the fabrication of semiconductor devices.
Claude Veillon et al., "Preparation of High-Purity Volatile Acids and Bases by Isothermal Distillation", Anal. Chem. vol. 53, pp. 549 (1981), discloses a preparation of volatile acids and bases of high purity by evaporating the mixture of chemical substances and highly purified water. However, the process is not applicable to acids and bases of low volatility.
Vacuum distillation may be used in distilling acids of high vapor pressure, but it is not applicable to microanalysis because of possible further contamination of the sample from the wall of the sample container during heating.
Sub-boiling systems are widely applied for concentrating an acid. These systems directly heat the chemical substances to increase the vapor pressure of the acid. The vapor is subsequently cooled in a collection area where the ambient temperature is lower than the boiling point of the acid. These systems are designed primarily for purity of the collected liquids; generally, they are not designed to prevent further contamination of the sample of chemical substances.
FIG. 1 is an example a sub-boiling system as described in Anal. Chem. vol. 44, No. 9, pp. 1716 (1972). The sub-boiling system is a two-bottle TEFLON still system comprising a sample container 111, a collecting container 112, a connection block 113 for connecting the two containers 111 and 112, a heater 115 such as an infrared lamp for heating the sample container 111, and a cooling bath 114 for cooling the collecting container 112. The collecting container 112 is emerged in the bath 114 so as to condense the vapor generated from the sample container 111. This system may be practicable in concentrating acids of low boiling point such as fluoric acid, but is not suitable for acids of high boiling point such as sulfuric acid.
Another sub-boiling system, which is disclosed in R. W. Dabeka et al., "Polypropylene for the Sub-Boiling Distillation and Storage of High-Purity Acids and Water", Anal. Chem. vol. 44, No. 8, pp. 1204 (1976), is shown in FIG. 2. It includes a sample container 211, a collecting container 212, a heater 215, a condenser 221, a collecting plate 222 fixed adjacent to the condenser 221, and a sample inlet 225. The collecting plate 222 is installed in the sample container 211 and connected to the collecting container 212. In this system, chemical substances are introduced into the sample container 211 through the sample inlet 225 and the sample container 211 is heated by the heater 215 to vaporize the chemical substances. The vaporized chemical substances are cooled in the condenser 221 and collected in the collecting container 212. This system is effective in distilling acids of relatively high boiling point such as sulfuric acid, but the system suffers some disadvantages. Too much time is required to concentrate the sample such that the sample may become contaminated. Also the system is not applicable to microanalysis due to its constructional complexity.
In Edwin C. Kuehner et al., "Preparation and Analysis of Special High-Purity Acids Purified by Sub-Boiling Distillation", Anal. Chem. vol. 44, No. 12, pp. 2051 (1972), a quartz device for still another sub-boiling system is disclosed. This quartz device is shown in FIG. 3, and includes a sample container 311, a collecting container 312, a heater 315, a condenser 321, a collecting plate 322, a liquid regulator 331, a sample feed tube 332, an evaporating container 333, and a sample outlet 334. The sample container 311 is connected to the evaporating container 333 through the liquid regulator 331 and the sample feed tube 332, and the heater 315 is provided in the evaporating container 333.
First, chemical substances in the sample container 311 are supplied to the evaporating container 333 through the liquid regulator 331 and the sample feed tube 332, and heated by the heater 315 so as to generate their vapors. The vaporized chemical substances are cooled in the condenser 321 and the resulting liquid is collected by the collecting plate 322. Then, the liquid flows to the collecting container 312 through the sample outlet 334.
This system may be applicable to acids of high boiling point such as sulfuric acid, but it also suffers drawbacks such as requiring too much time for concentrating the sample of chemical substances, contaminating the concentrated sample when the sample is moved to an analytical instrument, and not being applicable for microanalysis due to its complex construction.
In still another method of concentrating chemical substances, a quartz beaker containing the sample is placed in a Pyrex box. Nitrogen gas is passed through a HEPA filter and into the box. The sample is heated by irradiating the Pyrex box with external high-powered infrared rays originating from above the box. In addition, the sample can be heated with a hot plate provided in the bottom of the box. This method may be useful for concentrating acids with high boiling points without the burden of an air conditioner in a laboratory, but it suffers from loss of sample through the wall of the beaker and possible contamination of the sample caused by the direct heating of the Pyrex box or quartz beaker.
U.S. Pat. No. 4,263,269, discloses a method of obtaining an aqueous acid without contaminants by contacting the contaminated aqueous acid with a counter-flowing vapor. This method is not applicable to the concentration of a sample of chemical substances as a pretreatment for quantitative and qualitative analyses of the sample.
A need exists, therefore, for a system that will remove acids from a sample of chemical substances, thus concentrating the sample, without introducing further contaminants to the sample. In addition, there is a need for a system that will remove a wider range of acids more quickly than current systems. Furthermore, there is a need for a system with constructions that are not too complex for application to microanalysis.